This invention relates to the manufacture of shaped articles from flowable polymerizable resin-forming compositions employing reaction injection molding apparatus and techniques.
Molding resins are broadly classified as thermoplastic or thermosetting and molding apparatus and techniques vary considerably according to which of these two categories of materials is being processed. Thermoplastic molding resins, for example, polyolefins, polyvinyls, polyamides, polycarbonates, polystyrenes, etc., are, when heated to the plastic state, highly viscous fluids, e.g., they possess viscosities of from about 10,000 to about 50,000 centipoises or more, and injection pressures of a considerable magnitude, e.g., from about 500 to about 5,000 p.s.i.a., and in some cases even higher, are required to completely fill the mold cavity and expel the bulk of any air and/or other gas from the mold cavity through vents provided for this purpose, generally small grooves in the parting line of the mold extending from the lip of the mold cavity to its outer edge. The gas, being far less viscous than the thermoplastic, is readily displaced from the mold cavity under the influence of the high injection pressures whereas the thermoplastic is fully retained in the mold cavity. The high injection pressures which are typical of thermoplastic injection molding operations also serve to maintain the small quantities of gas which are inevitably present in the molding resins in solution thereby preventing the formation of bubbles or voids in the molded product.
Reaction injection molding (RIM) apparatus and processes have been gaining increasingly wider acceptance for manufacturing shaped articles from thermosetting resins, largely because of the low injection pressures and shorter molding cycles of these resins compared to those for thermoplastic resins but also due to the fact that the excellent physical and chemical properties of many thermosetting resins, e.g., polyurethane elastomers, make them attractive candidates for numerous engineering applications such as automotive body panels and bumpers, vibration dampeners, gaskets, power transmission belts, conveyor belts, and the like.
A typical flowable polyurethane resin-forming composition is prepared by the high-speed mixing of an isocyanate-reactive liquid polymer such as a polyether or polyester glycol, triol or tetrol, amine-terminated polyether, hydroxyl-terminated polybutadiene, etc., and mixtures thereof, an organic polyisocyanate, a chain extender, and, optionally, one or more other components such as catalyst, filler, colorants, etc., to provide a homogeneous liquid of relatively low viscosity, e.g., from about 50 to about 10,000 centipoises, and preferably from about 500 to about 5,000 centipoises. The composition is then injected into the cavity of a mold, which may or may not be heated, in as brief a time following mixing of the aforesaid components as possible in order to prevent any significant degree of gelation from taking place before the full amount of reaction medium required has been introduced into the mold cavity. Injection pressures are generally quite low, usually from about 2 to about 120 p.s.i.a., since appreciably higher pressures would only force low viscosity reaction medium through the mold vents. For a more detailed discussion of reaction injection molding apparatus and processes, reference may be made to Lloyd et al., "Polyurethane RIM: A competitive Plastics Molding Process", ACS Symposium Series 270 and W. E. Becker, eds., "Reaction Injection Molding", Van Nostrand Reinhold Co., New York, 1979.
In practice, it is just about impossible to exclude air or other gas from liquid thermosetting resins due to the chemistry of the polymerization process (gelation followed by curing). Taking the specific case of polyurethane elastomer-forming compositions, there are several sources of gas all of which can, and do, prove troublesome in the molding process. All polyurethane reaction media must contain a polyisocyanate component. Polyisocyanates are very reactive toward substances containing active hydrogen which, of course, includes water. In the manufacture of polyurethane foams, it is the reaction of polyisocyanate with water which, in a train of chemical events, results in the evolution of carbon dioxide gas. The foaming, or blowing, activity of carbon dioxide may or may not be supplemented by an auxiliary blowing agent such as a low boiling halogenated hydrocarbon. In contrast to polyurethane foam reaction media, polyurethane elastomer-forming compositions must scrupulously exclude water and water vapor in order to avoid the generation of carbon dioxide gas. When the elastomer is polyester polyol based, it is practically impossible to completely remove all traces of water resulting from the manufacture of the polyester. Another source of gas in polyurethane reaction media is the nitrogen blanket which is employed for the purpose of preventing contact of each reaction component with atmospheric water vapor during storage. While very little of the gas remains after the reaction components have been passed through a degasser, there may still be enough nitrogen present to significantly contribute to the overall problem of gas entrapment during molding. Still another source of gas results from the manner in which the polyurethane reaction media are injected into the mold. During the injection cycle, air is unavoidably introduced into the reaction media stream. Moreover, any turbulence or "splashing" effect accompanying the filling of the mold cavity can introduce further quantities of air into the reaction media.
The presence of gas bubbles or voids in a molded polymeric article is highly undesirable for several reasons. Those present at the surface of the article detract from its appearance, a defect which is often sufficient of itself to result in failure of the article to meet minimum quality standards. Those within the interior of the article can significantly diminish its physical and mechanical properties limiting not only the ability of the article to perform as well as desired but reducing its useful service life as well. For example, in the case of a polyurethane elastomer article which experiences frequent compression-decompression cycles, e.g., a vibration dampening device based on this resin, the "dieseling" effect which can occur within bubbles of entrapped gas can cause localized hot spots within the article which accelerate its failure.
Design and process variables in a reaction injection molding system can be controlled to some extent to reduce the amount of air and/or other gas entrapped within liquid resin-forming media. However, since the viscosities of flowable thermosetting reaction media are generally too low to permit the use of injection pressures which would maintain entrapped gas in solution, e.g., the relatively high levels of injection pressure which are commonly used in thermoplastic molding operations, it is not possible to eliminate or suppress the problem of entrapped gas bubbles or voids in reaction injected molded products by resort to high injection pressures.
According to U.S. Pat. No. 3,853,446, the contents of which are incorporated by reference herein, a reaction injection molding apparatus is disclosed wherein a movable piston, or "bottom plate", which forms part of the mold cavity is actuated immediatelY following the injection of a liquid resin reaction medium into the cavity, i.e., before any appreciable increase in viscosity of the reaction medium (due to polymerization) has taken place. Upward movement of the bottom plate by a predetermined distance simultaneously closes the runner (the passage through which the liquid reaction medium is introduced into the mold cavity) and reduces the volume of the mold cavity to the point where the reaction medium completely fills the mold. It is to be noted that the bottom plate does not act upon a reaction medium of appreciably increased viscosity nor does it subject the reaction medium to any pressure significantly above that which is used in the filling of the mold.
While the unique means provided in U.S. Pat. No. 3,853,446 for introducing a polymerizable liquid resin-forming composition into the cavity of a reaction injection mold through the geometric center, or "point of balance", of the bottom plate can greatly reduce the turbulence within the mold during the injection cycle, this in itself may not always be enough to prevent entrapment of an excessive quantity of gas and consequent formation of an excessive number of gas bubbles or voids in the molded articles.
In addition to the problem of entrapped gas bubbles or voids, some types of liquid resin-forming reaction media, e.g., acrylics, unsaturated polyesters, and the like, are susceptible to yet another problem, namely, volume shrinkage, which occurs during their polymerization. Shrinkage causes such polymerizing reaction media to pull away from mold cavity surfaces resulting in a molded article with rough-textured surfaces.
In the so-called "cell casting" method of manufacturing transparent poly(methylmethacrylate) sheet, e.g., as described in U.S. Pat. Nos. 2,347,320, 2,687,555 and 2,867,003, wherein shrinkage in the direction of thickness on the order of about 21% is fairly typical, sufficient clamping pressure is maintained on the sealed, fluid-tight molding cell, a pair of glass plates separated by a compressible gasket, to prevent the polymerizing contents of the cell from pulling away from the plates. While this approach to compensating for in-mold volume shrinkage is quite effective, it is unsuitable for reaction injection molding operations which employ vented molds. Placing the contents of a sealed, but vented, reaction injection mold under relatively high pressure before the contents of the mold have had an opportunity to attain a level of viscosity which would prevent their being forced through the mold vents would not be a practical solution to the problem of volume shrinkage in reaction injection molding operations.
Reaction injection molding apparatus and processes in which increased pressure is applied to a polymerizing reaction injection molding composition only after the composition has reached some predetermined level of gelation are known from European Patent Application No. 24,610; German Offenleg. DE 3,522,377 Al; Ruhmann et al., "Aspekte zur Konstruktion von RIM-Anlagen der Zukunft", Kunstoffe, Carl Hanser Verlag, Munich, Germany (1987); Boden et al., "New Possibilities of Improving the RIM Process by Utilizing Externally Applied Holding Pressure", Polyurethane World Congress 1987 (Sept. 29 to Oct. 2, 1987); and, "Why Horizontal RIM Looks Hot", Plastics Technology, pp. 31 et seq. (May, 1988). However, in all cases, the increased pressure is applied in response to a time-programmed signal the actuation of which assumes that a certain minimum level of gelation of the polymerizing contents within the mold has been reached. Thus, for a specific liquid resin-forming composition, the viscosity build-up characteristics of the composition as a function of time under a given set of polymerization conditions are experimentally determined and the reaction injection molding apparatus is programmed accordingly to apply a predetermined level of increased pressure at a time corresponding to a desired point on the experimentally determined viscosity build-up curve. This approach to applying "after-pressure" in a reaction injection molding operation, while often effective, can lead to unacceptable results when the moment the after pressure is applied does not, in fact, correlate with the level of viscosity, or degree of gelation, assumed to have been reached. Unpredictable fluctuations in ambient temperature, mold temperature and other process variables which can be difficult to manage can render the time-programmed application of after-pressure an ineffective or unreliable technique for avoiding or minimizing the formation of entrapped gas bubbles or voids in reaction injection molding operations and/or compensating for in-mold volumetric shrinkage.